\[
\begin{array}{l}
 \left( \begin{array}{l}
 n \\ 
 0 \\ 
 \end{array} \right) - \left( \begin{array}{l}
 n \\ 
 2 \\ 
 \end{array} \right) + \left( \begin{array}{l}
 n \\ 
 4 \\ 
 \end{array} \right) - \left( \begin{array}{l}
 n \\ 
 6 \\ 
 \end{array} \right) + ... = \sqrt {2^n } \cos \frac{{n\pi }}{4} \\